A conventional excavating-and-slewing working vehicle includes respective hydraulic cylinders for driving working machines of a boom, an arm, a bucket and a bulldozing blade, a hydraulic cylinder for swinging a boom bracket, a hydraulic motor for slewing a main body of the vehicle, and a pair of right and left hydraulic motors for traveling, which are supplied pressure oil from a plurality of hydraulic pumps attached to an engine. Three or more hydraulic pumps are attached to a large-sized excavating-and-slewing working vehicle. A small-sized excavating-and-slewing working vehicle generally includes only two pumps because there is no space for juxtaposing many pumps in a small bonnet thereof. The actuator driving system of three pumps is called “three pump system”, and that of two pumps is called “two pump system”.
One of the hydraulic pumps of the two pump system may drive the hydraulic cylinder for boom, arm or the like together with the hydraulic motors for traveling simultaneously. Thus, if the boom, arm or the like is driven during traveling of the vehicle, either the hydraulic motors for traveling or the hydraulic cylinder, or both of them cannot be driven fully because the amount of pressure oil thereto is insufficient.
Japanese Patent No. 2,760,702 and Japan Patent Application Laid Open Gazette Hei. 10-195933 (sic), for example, disclose that pressure oil delivered from the two hydraulic pumps is controlled so as to ensure a sufficient amount of pressure oil.
As disclosed in Japanese Patent No. 2,760,702, a left traveling hydraulic motor, a bucket cylinder and a boom cylinder are essentially driven by one of the hydraulic pumps, and a right traveling hydraulic motor, an arm cylinder and an external hydraulic apparatus are by the other hydraulic pump. Bypass oil passages branch from the upstream (sic) side of the respective control valves for the traveling hydraulic motors and are provided with respective check valves so that each hydraulic pump can supply pressure oil to actuators essentially driven by the other hydraulic pump. However, pressure oil flows through each of the branch points into a lower-pressure side so as to collapse the driving balance among the driven actuators. For example, the vehicle unexpectedly turns left or right during its traveling.
Furthermore, this disclosed hydraulic circuit requires three parallel hydraulic passages. If the directional control valves are aligned in a stratified form so as to constitute a compact valve device, it is difficult for the valve device to make a space for arranging three common oil passages therein.
An art disclosed in Japan Patent Application Laid Open Gazette Hei. 10-195933 (sic) solves the problem of an excavating-and-slewing working vehicle that, when the boom is operated during traveling of the vehicle, a pressure difference may be is generated between the right and left traveling motors so as to disturb the translatory movability of the vehicle. A boom control valve is tandem-connected to the downstream side of the right and left traveling control valves. The two hydraulic pumps are connected at output sides thereof to each other through a bypass passage at the upstream side of the right and left traveling switching valves. The bypass passage is connected to the boom control valve through an orifice for pressure compensation.
However, only the boom actuator can be driven simultaneously with traveling drive of the vehicle without disturbing translatory movability. When the actuators for slewing, arm, bucket and PTO are driven during traveling of the vehicle, the vehicle cannot secure translatory movability, or unexpectedly turn left or right during traveling. Further, when working actuators are driven simultaneously, the activity of the driven actuators is unsatisfactory.
Furthermore, even if the vehicle excavates in the state of being stationary, there is impossible simultaneous operation of work parts for the conventional excavating-and-slewing working vehicle of two pump system. A general excavation cycle and the above-mentioned accompanying motions of actuators by an excavating-and-slewing working vehicle are shown in FIG. 2, which will be discussed in the later description of the present invention.
The excavation cycle comprises three stages, that is, excavation, soil-removal, and return-and-location. When work is started, a boom is moved downwardly and an end portion of a bucket is hit to the ground, and an arm and a bucket are simultaneously operated for excavation. Next, simultaneously with drive of the boom, a slewing body provided above a crawler type traveling equipment is slewed so that the vehicle turns to a side with the bucket holding soil, and the bucket is operated to damp the soil. Then, the arm and the slewing body are operated simultaneously, or the boom, the arm and the slewing body are operated simultaneously, so that the work machine is returned to the initial place and located.
As mentioned above, in the general excavation cycle of the excavating-and-slewing working vehicle, simultaneous operations of the arm and the bucket, of the boom and the slewing body, and of the arm and the slewing body or of the boom, the arm and the slewing body are performed.
The conventional three pump system has such a general construction as shown in FIG. 31(a) to supply pressure oil from the pumps to the actuators required for excavation. In this system, three pumps supply pressure oil to respective three actuators. Accordingly, as shown in FIG. 31(b), even if the boom, arm and slewing body are simultaneously operated, they obtain satisfactory activity.
On the other hand, the conventional two pump system has such a general construction as shown in FIG. 32(a) to supply pressure oil from the pumps to the actuators required for excavation. In this system, one pump supplies pressure oil to the slewing body and the arm, and the other to the boom and the bucket. The above-mentioned excavation cycle includes some operations requiring one pump to drive two actuators, e.g., simultaneous double operation of the arm and slewing body and simultaneous triple operation of the boom, arm and slewing body. Therefore, as shown in FIG. 32(b), two pump system is inferior to three pump system in some cases where two or more actuators are simultaneously operated. Thus, a current two pump system is adopted to only a small-sized excavating machine which does not consider high-workability seriously.